1. Field of the Invention
This invention relates to a method for preparing a water-impermeable, flexible or semi-hard, open cell polyurethane foam.
2. Prior Art
Several methods are known in the art for the preparation of low air-permeable, water-impermeable, flexible or semi-hard, open cell polyurethane foams. For example, (1) a flexible or semi-hard, open cell polyurethane foam is impregnated with a volatile solution of asphalt followed by drying. (2) A sealing material in the form of a flexible or semi-hard, open cell polyurethane foam is prepared by combining at least one of a polydienic polyol, a dimer acid polyol, and a polyol having more than 90 mol % of an alkylene oxide addition polymerized thereto with a hydroxyl-containing organosilicone as disclosed in Japanese Patent Publication (JP-B) No. 55470/1990. (3) Japanese Patent Application Kokai (JP-A) No. 27583/1988 discloses a method for preparing a sealing material in the form of a polyurethane foam having a polyolefin skeleton as a polyol. (4) JP-A 298574/1990 discloses a method for preparing sealing material in the form of a saturated hydrocarbon resin skeleton polyurethane foam using a hydroxyl-containing reactive silicone.
These methods have the following drawbacks. Method (1) suffers from the problems of environmental pollution by the solvent given off during the drying step, foam productivity, and collapse of once expanded cells. Method (2) has no outer appearance problem if the foam density is as high as 40 kg/m.sup.3, but in a low density region, suffers from the occurrence of void-like pinholes or cracks within the foam. Methods (3) and (4) are not economical.
Often, conventional flexible or semi-hard, open cell polyurethane foams are generally prepared by using a polyether or polymeric polyol mainly containing a propylene oxide with a molecular weight of 450 to 8,000, tolylene diisocyanate, a blowing agent, and a foam stabilizer. Depending on particular types of polyol and amine catalyst as well as their amounts, low air-permeable foams can be prepared in a relatively simple manner. However, it is not expectable that the resulting foams are also impermeable to water.
JP-A 245623/1985 discloses a flexible polyurethane foam having low air permeability which is prepared from a blend containing a polyether polyol and a carboxyl-modified organosilicone. This foam is less impermeable to water. Furthermore, JP-A 38152/1989 discloses to use as the foam stabilizer an organosilicone compound which is based on a polydialkylsiloxane having a primary or secondary amino group. This foam is also less impermeable to water.